1. Field of the Invention
The present invention relates to a process for producing a magnetic recording medium comprising a non-magnetic support and a magnetic layer.
2. Description of Prior Art
As magnetic recording media for computers, video-tapes and audio tapes, there have been used magnetic recording mediums having a magnetic layer on a non-magnetic support.
Such magnetic recording media have been produced by steps including a step wherein a non-magnetic support is coated with a magnetic paint comprising magnetic layer forming components such as a ferromagnetic powder and a resin component dissolved or dispersed in an organic solvent, and the organic solvent in the coated layer is then removed.
Recently, demands for higher-density recording have been desired, and finely divided ferromagnetic powders have been used for the magnetic recording medium.
Generally, when a finely divided ferromagnetic powder is used, electromagnetic conversion characteristic is improved. However, the finer the powder, the dispersibility of the ferromagnetic powder in the magnetic paint becomes poorer. Accordingly, there is a possibility that the squareness ratio of the magnetic layer of the resulting magnetic recording medium is insufficiently improved, or the surface of the magnetic layer is made poorly smooth. Hence, there is a problem that through the finely divided ferromagnetic powder is used for the purpose of improving electromagnetic conversion characteristics, the electromagnetic conversion characteristics of the resulting magnetic recording medium can not be improved to the expected degree.
Generally, a magnetic paint is prepared by adding a small amount of an organic solvent to a magnetic layer forming component comprising a ferromagnetic powder, a resin component, etc., kneading the mixture and dispersing the kneaded mixture in a large amount of an organic solvent.
It has been previously thought that the characteristics of a magnetic recording medium are hardly affected by an organic solvent which are used for the preparation of the magnetic paint. Accordingly, there have been used organic solvents which are inexpensive and have boiling points within a moderate range, such as methyl ethyl ketone, methyl isobutyl ketone and cyclohexanone in the kneading and dispersion steps. So far as the present inventors know, the same organic solvents are used in the kneading and dispersion steps.
The present inventors have studied and found that in the step of kneading the ferromagnetic powder and the resin component, there are differences in the amount and the state of the resin component to be fed to the surface of individual particles of the ferromagnetic powder depending on the types of the organic solvents. It has been also found that the electromagnetic conversion characteristics of the magnetic recording medium are affected by such differences. Accordingly, other consideration is needed in the use of an organic solvents which are used for the production of the magnetic recording medium.